Condensed Consolidating Information (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Condensed Consolidating Financial Statements
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 287,120	$ 420,938	$ 390,761	$ 456,694	$ 277,691	$ 186,513
Investments in short-term commercial paper			84,830
Accounts receivable, net	712,750		722,791
Inventories	1,209,079		1,199,584
Other current assets	53,275		57,292
Total current assets	2,262,224		2,455,258
Property, plant and equipment, net	2,213,703		2,193,745
Intangible assets, net	425,034		450,893
Goodwill	740,192		745,066
Other assets, including investments in subs	134,363		134,264
Total assets	5,775,516		5,979,226	6,003,481
Accounts payable	377,213		420,824
Accrued expenses	198,072		235,515
Current maturities of long-term debt	30,114		444,078
Total current liabilities	605,399		1,100,417
Long-term debt	2,173,925		1,936,022
Other liabilities	563,364		572,193
Redeemable noncontrolling interest	93,774		70,694
Common stock	636		636
Treasury stock	(720,479)		(722,653)
Additional paid-in capital	1,033,857		1,026,157
Retained earnings (deficit)	2,049,011		2,011,801
Total Steel Dynamics, Inc. equity	2,363,025		2,315,941
Noncontrolling interests	(23,971)		(16,041)
Total equity	2,339,054		2,299,900
Total liabilities and equity	5,775,516		5,979,226
Parent
Condensed Consolidating Financial Statements
Cash and equivalents	242,963		301,073	388,196		173,563
Investments in short-term commercial paper			84,830
Accounts receivable, net	275,227		319,995
Inventories	607,567		584,079
Other current assets	57,564		83,116
Total current assets	1,183,321		1,373,093
Property, plant and equipment, net	1,020,336		1,059,011
Other assets, including investments in subs	2,768,937		2,791,718
Total assets	4,972,594		5,223,822
Accounts payable	125,105		155,220
Accrued expenses	120,827		157,868
Current maturities of long-term debt	14,227		439,199
Total current liabilities	260,159		752,287
Long-term debt	2,144,521		1,905,199
Other liabilities	204,889		250,395
Common stock	636		636
Treasury stock	(720,479)		(722,653)
Additional paid-in capital	1,033,857		1,026,157
Retained earnings (deficit)	2,049,011		2,011,801
Total Steel Dynamics, Inc. equity	2,363,025		2,315,941
Total equity	2,363,025		2,315,941
Total liabilities and equity	4,972,594		5,223,822
Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	26,990		58,699	59,517		10,628
Accounts receivable, net	832,486		726,192
Inventories	499,059		533,022
Other current assets	13,384		4,119
Total current assets	1,371,919		1,322,032
Property, plant and equipment, net	654,436		660,048
Intangible assets, net	425,034		450,893
Goodwill	740,192		745,066
Other assets, including investments in subs	31,405		33,507
Total assets	3,222,986		3,211,546
Accounts payable	245,287		273,049
Accrued expenses	102,065		98,498
Current maturities of long-term debt	300		300
Total current liabilities	347,652		371,847
Other liabilities	2,104,391		2,132,778
Common stock	33,896		33,896
Additional paid-in capital	117,737		117,737
Retained earnings (deficit)	619,310		555,288
Total Steel Dynamics, Inc. equity	770,943		706,921
Total equity	770,943		706,921
Total liabilities and equity	3,222,986		3,211,546
Combined Non-Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	17,167		30,989	8,981		2,322
Accounts receivable, net	11,238		8,971
Inventories	109,038		85,046
Other current assets	3,778		2,304
Total current assets	141,221		127,310
Property, plant and equipment, net	541,605		477,514
Other assets, including investments in subs	9,383		9,853
Total assets	692,209		614,677
Accounts payable	75,072		23,195
Accrued expenses	10,418		10,012
Current maturities of long-term debt	34,313		53,551
Total current liabilities	119,803		86,758
Long-term debt	173,260		169,797
Other liabilities	38,512		51,625
Redeemable noncontrolling interest	93,774		70,694
Common stock	18,121		18,121
Additional paid-in capital	453,287		347,151
Retained earnings (deficit)	(180,577)		(113,428)
Total Steel Dynamics, Inc. equity	290,831		251,844
Noncontrolling interests	(23,971)		(16,041)
Total equity	266,860		235,803
Total liabilities and equity	692,209		614,677
Consolidating Adjustments
Condensed Consolidating Financial Statements
Accounts receivable, net	(406,201)		(332,367)
Inventories	(6,585)		(2,563)
Other current assets	(21,451)		(32,247)
Total current assets	(434,237)		(367,177)
Property, plant and equipment, net	(2,674)		(2,828)
Other assets, including investments in subs	(2,675,362)		(2,700,814)
Total assets	(3,112,273)		(3,070,819)
Accounts payable	(68,251)		(30,640)
Accrued expenses	(35,238)		(30,863)
Current maturities of long-term debt	(18,726)		(48,972)
Total current liabilities	(122,215)		(110,475)
Long-term debt	(143,856)		(138,974)
Other liabilities	(1,784,428)		(1,862,605)
Common stock	(52,017)		(52,017)
Additional paid-in capital	(571,024)		(464,888)
Retained earnings (deficit)	(438,733)		(441,860)
Total Steel Dynamics, Inc. equity	(1,061,774)		(958,765)
Total equity	(1,061,774)		(958,765)
Total liabilities and equity	$ (3,112,273)		$ (3,070,819)